I know there might be many peculiarities that guys might never understand about girls. But seriously, why do guys need a separate toilet for peeing? I'm sure when guys are in your own home, they use the normal toilet bowls, not urinals. Why is it that when you guys go to the public bathroom, you suddenly switch to something else? Is it because the option is there?

Big reasons why not to use urinals: Privacy
I've seen in many movies how each urinal in the guy's bathroom are lined up directly next to each other. Everything is out in the open. There is absolutely no privacy. People can actually see you peeing while they are walking by. The best you can expect is a long plastic panel that sticks out to separate each stall. Even with that, people can still see you peeing. I think this reason alone would make anyone want to use the locked stalls. But they don't and for some reason, guys sometimes complain about how other guys decide to pee right next to them. Why didn't you just use the locked stall instead? You do it at home. At first I thought it was sanitary issues but honesty, how bad can your aim be that you need a specialized toilet?

Hypotheses:
-more manly to pee standing up, even more manlier to show it off....therefore, urinals
-people will think they are peeing sitting down if they use the locked stall, guys find this embarrassing...therefore, urinals
-peeing in the toilet stall is similar to that of a women, men find it insulting....therefore, urinals
-Since they are not in their own home, guys think it's fun to graffiti with their urine....therefore, urinals
-It's dirty to pick up the seat in the public bathroom...therefore, urinals

I think you're way off on this one. You clearly haven't shared a bathroom with men/man/boy(s). Urinals exist because they pee on the floor and can't get it all in the toilet unless they sit down, which they are unwilling to do because everyone else has been pissing all over the toilet (or worse).

I have two bathrooms in my condo. My son and cat share a bathroom because they are both disgusting and I can't stand the smell after a day or two.

Also, men don't care as much about privacy as you think, especially since they can hide most of it with their hands. It's not like they have to show their asses while they do it or anything.

Edit: Oh yeah, and my son wouldn't stop peeing out the front door until we moved to this condo (with a shared hallway). He was too lazy to walk another 30 feet to the back door, or 50 feet to the bathroom. This is not something I taught him, and from what I hear it's not uncommon, even in full-grown men.

It is much more comfortable to pee while standing too. I have actually learned recently that girls don't seem to get those pee satisfaction chills and it is a guy thing. I believe it may be related to peeing while standing but sometimes when you have been holding it for awhile and you let it all out, you get a chill of satisfaction throughout your body either during or just after you whizzed.

Some urinals now have wall separators for privacy but I don't think guys care all that much anyway. Showers in change rooms always seem to be common for the men's side and individual for the ladies sides. At east where I grew up anyhow.

For the privacy thing, it depends on the person. I need my buffer space.. I don't like anyone being that close to me.

As for.. why urinals? Like another has said.. because guys tend to stand when they piss and they get it everywhere.
So urinals allow those who need to shit the ability to without having to spend 10 minutes cleaning pee from everything.

Utility is the easiest reason, in my opinion. If you you only have to urinate, then you don't have to sit down. That's why there are often more urinals than toilet seats in many men's bathrooms. Without getting too graphic, and thus getting the thread locked, guys can get their streams split, and it's not something they can necessarily control. Sitting down helps deal with that, but refer back to my first sentence here.

The privacy issues you mention are more psychological in nature, and not all guys are, for lack of better phrasing, weirded out by someone standing next to them. It might be different if the other guy was, say, actively staring (but some guys are into being stared at, so there's also that...) Point is, the outlook guys have isn't universal, and some of the scenarios you describe are played out more for comedic effect than a reflection of every guy's thoughts.

Have you ever peed while standing up? If you haven't experienced this feeling of wonder- I mean perspective, then all of your comments are biased. Besides, we're all guys in there- unless a father brings in his under 5 year old daughter. We don't have to worry about seeing each others- actually, why would a guy even look there? And your statement about how easy it must be to aim correctly- it is harder than your think! And how are urinals without stalls worse than communal baths? If you were a guy, hypothetically speaking, and you were out in the woods and really had to pee, would you go sitting down or standing up? You'd pick the most convenient choice- standing. For girls, technically you can pee while standing, but it might get messy. Plus, when a girl has to take care of... certain things... it's best to do it in a stall, rather than standing up. If you go to other countries, they have their own style urinal- not just the ones standing up. I guess it makes things easier- no seat to clean, less chance of clogging, less chance of some stupid guy stuffing toilet paper in it for fun, quicker, and I think men don't care as much about the process of using the bathroom as much as women, on a general sense.

Me personally, I hate to walk into a stall and see the person before me left the toilet seat up and cared 0% about how the next person would use the toilet, therefor- urinals.

Why would I want to open another door when the urinal is right there out in the open. Not to mention there tends to be surprises waiting for you in the regular toilet stalls. I don't care if people stare at my wiener why I piss, Its not there to impress them. Plus, stuff splashes out of regular toilets when you stand and pee in them. I also do not care if urinals use less water and are cheaper, its just way more convenient all around.